1. Field of the Invention
The present invention relates to methods for producing metallic iron, and more particularly, relates to an improved method for producing metallic iron. According to the improved method described above, when the metallic iron is produced by the steps of feeding a mixture, which contains a carbonaceous reductant such as coal and iron oxide such as iron ore, onto a moving hearth of a moving hearth reducing-melting furnace, then reducing and melting the iron oxide by heating, and subsequently cooling the metallic iron thus obtained for production, a continuous operation can be stably performed while damage done to the moving hearth is reduced as small as possible, or damaged surface portions of the hearth is repaired during operation.
2. Description of the Prior Art
Among methods for producing reduced iron, the following method has been known. In the method mentioned above, after a mixture of iron oxide such as iron ore and a carbonaceous reductant such as coal is fed onto a hearth of a rotary furnace or a moving hearth reducing furnace such as a straight grate and is then heated by radiant heat in the furnace while being moved therethrough so that the iron oxide is reduced by the carbonaceous reductant to produce reduced iron, the reduced iron thus produced on the hearth is discharged outside the furnace by optional discharge means such as a screw mechanism.
However, when the mixture is composed of agglomerates in the form of pellets or the like, and the agglomerates as mentioned above are fed onto the hearth, powders are generated from the agglomerates by dropping impact or the like and are then deposited on the hearth. Since the powders thus deposited are heated and reduced together with the agglomerates, the deposited powders are formed into powdered reduced iron, and the agglomerates are formed into granular reduced iron. The granular reduced iron is discharged outside the furnace by a discharge screw; however, on the other hand, the powdered reduced iron is squeezed into the surface of the hearth. Accordingly, when a continuous operation is performed, the amount of the powdered reduced iron squeezed into the surface of the hearth is increased, and in addition, the powders of the reduced iron are bonded to each other by a compression force of the discharge screw; hence, a problem has occurred in that plate-shaped reduced iron is formed on the surface of the hearth. In the case of a moving hearth reducing furnace, since a heating and reducing zone is at a high temperature, and a raw material-feeding zone and a discharge zone are at a relatively low temperature, cracking or warping of the plate-shaped reduced iron formed on the surface of the hearth is likely to occur because of the temperature differences described above. In addition, when the plate-shaped reduced iron is caught by a discharge screw, troubles such as shutdown thereby occur.
The inventors of the present invention have already proposed (U.S. Pat. No. 3,075,721) a technique that solves the problems described above. In the technique mentioned above, a hearth-forming material is fed into a furnace together with agglomerates, powders generated from the agglomerates are deposited on a surface of a hearth to form an iron oxide layer thereon, and in addition, a discharge device is intermittently or continuously moved in the direction toward the top of the furnace so that the operation is carried out while the gap between the discharge device and the iron oxide layer formed on the surface of the hearth is adjusted. Accordingly, powdered reduced iron is prevented from being squeezed into the surface of the hearth by the discharge device, the formation of plate-shaped reduced iron on the hearth is prevented thereby, and the deposited reduced iron powders are periodically scraped off; hence, a continuous operation can be carried out. The continuous operation can be carried out since the surface of the hearth is periodically renewed and repaired by scraping off the plate-shaped reduced iron formed on the surface of the hearth; however, the hearth itself is not scraped off.
In addition, as a method for producing metallic iron, a production method has been known in which after a mixture of iron oxide and a reductant is fed into a moving hearth reducing-melting furnace such as a rotary hearth furnace and is heated by radiant heat in the furnace while moving therethrough so that the iron oxide is reduced by the reductant, and subsequently, carburization, melting, aggregation, and slag separation are performed, granular solid metallic iron is discharged outside the furnace after cooling and solidification. For example, the inventors of the present invention has proposed a technique disclosed in Japanese Unexamined Patent Application Publication 2000-144224 in which damage done to a hearth caused by molten iron is prevented by forming a vitreous hearth layer composed of iron oxide, carbon, and a silica compound on the surface of the hearth of a rotary hearth furnace. However, when the operation is continuously performed, the vitreous layer is degraded by slag infiltration and corrosion, and hence improvement has been still required for performing a stable and continuous operation.
The present invention was made in consideration of the circumstances described above, and an object of the present invention is to provide a method for producing metallic iron, the method being suitable for performing a long continuous operation. According to the method described above, even when powdered metallic iron is squeezed into a surface of a hearth, or the hearth is damaged by slag infiltration and corrosion, the removal and repair can be easily performed, and the operation rate and maintainability of the hearth can be improved.